This invention relates to cam phase adjustment in an internal combustion engine, and more particularly to a method of bounding the cam phase based on the requested engine power output.
Engine cam phase adjustment can be utilized to improve engine exhaust gas emissions and fuel economy. Typically, the cam phase (intake and/or exhaust) is adjusted to increase the overlap of the intake and exhaust valve openings during low-to-medium load operation. This reduces the volumetric efficiency of the engine, but increases internal recirculation of engine exhaust gas. Emissions are improved because the recirculated exhaust gas tends to reduce the peak combustion temperature and pressure, and fuel economy is improved because the recirculated exhaust gas raises the intake manifold pressure, reducing engine pumping losses. Under high load operation, significantly higher inlet airflow is required, and the cam phase is shifted to a setting that maximizes the volumetric efficiency. Unfortunately, this control can noticeably degrade the power response of the engine on transitions from medium load to high load. Accordingly, what is needed is a cam phase control for achieving significant emission and fuel economy improvements without degrading the engine power response.
The present invention is directed to an improved cam phase control for an internal combustion engine in which the cam phase is scheduled to optimize emissions and fuel economy, but bounded for continuous achievement of the desired inlet airflow. According to the method, the volumetric efficiency required to achieve the desired inlet airflow at a specified intake manifold vacuum is compared to the maximum achievable volumetric efficiency at the specified vacuum. When the required volumetric efficiency is less than the maximum value, the cam phase is controlled according to the lesser of the scheduled cam phase and the highest cam phase for achieving the required volumetric efficiency; when the required volumetric efficiency is equal to or greater than the maximum value, the cam phase is controlled to the setting that maximizes the volumetric efficiency.